Origins
Meng Da had originally been Shuhan's Administrator of Yidu; at the end of the Jian'an era he surrendered to Wei out of fear after his failure to render aid during Guan Yu's defeat at Xiangfan. Cao Pi valued his talent and appointed him Administrator of Xincheng, overseeing the three commanderies of Fangling, Shangyong, and Xicheng — commanding the throat linking Hanzhong to Nanyang. After Cao Pi's death, Emperor Ming of Wei took the throne, and Meng Da lost his protector at court; as his old associates Huan Jie, Xiahou Shang, and others died off in succession, and he had long garrisoned a border commandery with no place to turn, he began to contemplate returning to his original lord. In the sixth year of Jianxing (228), when Shuhan launched the first northern expedition, Meng Da sent secret communications to Zhuge Liang through Li Yan's son Li Feng, pledging to lead the forces of the three commanderies of Xincheng, Shangyong, and Jincheng east to Luoyang and rendezvous with the Han army. Strategically, this was an imaginative "two-pronged assault": Kong Ming advancing on Qishan to pin down the Guanlong main forces, Meng Da cutting in from Nanyang into the flank and rear between Xu and Luo — had it succeeded, the Wei western front would have been torn open in a single blow.
Participating Forces
The rebel core consisted of Meng Da himself and his confidants Li Fu and his nephew Deng Xian — though these two were the very traitors who turned against him in the upheaval, revealing that Meng Da lacked control over his own closest associates. The ostensible allies, the brothers Shen Dan and Shen Yi, were local strongmen of Shangyong who nominally agreed to rise together, but in reality had already been secretly in contact with Wei's Shen Yi, "training troops daily while waiting for the Wei forces to arrive to serve as the inside agent". On the Shuhan side, Zhuge Liang — watching from across the mountains — had already, upon receiving the good news from Li Feng, anticipated Meng Da's inevitable failure; he wrote to warn Meng Da to stand strictly on guard, but Meng Da, convinced that "Kong Ming over-calculates," did not heed the warning. The Wei main force was Sima Yi commanding the Wancheng armies, with Xu Huang as vanguard, plus local forces loyal to the Wei court within Jincheng and Shangyong serving as the inside contingent.
Critical Turning Points
The first turning point in this case was the intelligence leak: a family member of Shen Yi came forward with a report on the secret plan, upon which Sima Yi completed his reconstruction of the full picture of "the Meng Da–Zhuge Liang coordinated operation" and immediately decided to "capture him first, causing Zhuge Liang's heart to grow cold and he will withdraw of his own accord". The second turning point was Sima Yi's decision to "act without waiting for the imperial edict": he judged that the one-month delay of memorializing, awaiting the edict, and returning would be enough to destroy the entire scheme, and so raised forces on his own authority, ordering double the standard daily march with death for any laggard; simultaneously he had his staff officer Liang Ji forge a "general's order" to dupe Meng Da into stockpiling grain. The third turning point was the vanguard Xu Huang's early arrival: Meng Da had originally calculated that Sima Yi's main force was still at Wancheng, at least a month's march from Xincheng, when suddenly Xu Huang's banners appeared beneath the city walls; the city's defenders rained arrows upon Xu Huang and drove him back wounded, but Sima Yi's great army then arrived to surround it completely on all sides, closing the time window in a single stroke. Finally, the inside work of Shen Dan, Shen Yi, Li Fu, and Deng Xian caused Meng Da to die in the disorder, and the entire revolt lasted only eight days.
Suppression and Outcome
After the revolt was suppressed, Sima Yi immediately marched back to Changan and joined forces with Zhang He moving east, preparing to respond to Zhuge Liang's advance at Jieting. Meng Da's head was sent to Luoyang for public display; the three commanderies of Xincheng, Shangyong, and Xicheng were reorganized, and the brothers Shen Dan and Shen Yi, having earned the first credit of reporting the plot, received appointments: Shen Yi was transferred to Administrator of Weixing, maximizing the political return of "bringing the whole commandery to submission." This disposition sent two signals: first, from this point forward Wei's evaluation of border generals gave markedly heavier weight to "political reliability"; second, a new form of contract between local strongmen and the court took shape — as long as one changed sides at the critical moment, prior contact with the rebel side could be pardoned.
Historical Resonance
Strategically, had Meng Da succeeded, the overall strategic picture of Shuhan's northern expeditions would have shifted from "single-line western advance" to "north-south pincer," potentially reducing the importance of Jieting enormously. For Sima Yi personally, this campaign was his first military action after his return from retirement in which he established his authority through "act first, report later" — it secured his political position in the mid-to-late Emperor Ming period of "frontier affairs all delegated to him," and provided the first installment of political capital for the type of action he would later employ at the Gaopinglingzhibian — acting on precise intelligence, not waiting for an imperial edict. Zhuge Liang's judgment of Meng Da — "Da is no match for Sima Yi and will certainly be captured" — became one of the "Kong Ming's discernment of men" cases that later historians and novelists alike have cited, forming a subtle counterpoint with the slightly later Zhanmasu case of "failure to discern men."